1. Field of the Invention
The present invention relates to a system for coupling a cooling fan of an internal combustion engine to a rotating member thereof such as a crank shaft, and more particularly relates to such a system in which the degree of rotational coupling between said cooling fan and said crank shaft is varied over three stages to respond to the cooling requirements for the engine, according to the temperature of the engine coolant and of the air passing through the engine radiator.
2. Description of the Prior Art
Various types of system for coupling an engine cooling fan to the engine crank shaft have been proposed. In particular, in Japanese Patent Applications Serial Nos. 59-18226 (1984) and 59-201379 (1984), neither of which is intended to admit as prior art to the present patent application except to the extent otherwise required by applicable law, applicant being the same as the assignee or entity owed a duty of assignment of the present patent application and which discloses a cooling fan fluid coupling system comprising a fluid circuit which is coupled to a member driven by the engine crank shaft to a member which drives the cooling fan, the amount of fluid circulation in said fluid circuit, which determines the degree of coupling provided by said fluid coupling system, being determined according to the opening amount of a first control valve and also according to the opening amount of a second control valve. The first control valve was controlled by a first actuator which was responsive to the temperature of the air passing through the vehicle radiator to cool it, and which incorporated a bimetallic spiral element or the like; while the second control valve was controlled by a second actuator which was responsive to the temperature of the coolant of the engine, and which incorporated a thermowax element or the like. Thereby, the amount of fluid circulation in the fluid circuit, which determined the degree of coupling provided by the fluid coupling system and hence for a given engine revolution speed the rotational speed of the fan and the amount of air sucked thereby through the radiator, was varied over three steps or stages, i.e. between low, medium, and high, according to the temperature of the air passing through said radiator and according to the temperature of the engine coolant.
In such engine cooling fan coupling system the control of switching over of the degree of coupling provided by said fluid coupling system is carried out in response to the temperature of the engine coolant and additionally in response to the temperature of the air passing through the radiator, so that the rotational speed of the fan better corresponds to the amount of cooling required for the engine, as compared to the conventional form of cooling fan coupling system in which the cooling fan is directly driven from the engine crank shaft, and even as compared to a form of cooling fan coupling system in which the control of switching over of the degree of coupling provided by the fluid coupling system is carried out in response only to the temperature of the engine coolant and not in response to the temperature of the air passing through the radiator. Nevertheless some problems still arise in practice. Specifically, since in such concept there is no particular coordination between the operation of a thermostatic valve which is typically fitted to the engine cooling system for controlling the flow of coolant therethrough and through the radiator, and the operation of the engine cooling fan coupling system, on occasion the engine cooling will not be properly carried out. As an example, if the temperature at which the above mentioned second control valve opens is set at a value considerably higher than the temperature at which the thermostatic valve opens, then the necessary cooling fan speed will not be attained, due to the excessive width of the domain over which said second control valve remains closed, and accordingly the coolant may reach an undesirably high temperature. If the temperature at which said second control valve closes is set at a value lower than the temperature at which said thermostatic valve is open, the second control valve, once open, will remain in the open condition without closing, even in an operational domain in which coolant temperature regulation is not specifically required, and accordingly the engine cooling fan will be operated at a high rotational speed unnecessarily, thus reducing engine power with no purpose, as well as increasing engine fuel consumption and cooling fan noise.